Positive opening and closing switch with wiping action



Aug. 30, 1955 E. G. FRANKLIN 2,716,682

POSITIVE OPENING AND CLOSING SWITCH WITH WIPING ACTION Filed Sept. 2,1953 2 Sheets-Sheet l g- 30, 1955 E. G. FRANKLIN 2,716,682

POSITIVE OPENING AND CLOSING SWITCH WITH WIPING ACTION Filed Sept. 2,1953 2 SheetsSheet 2 Tlg 4 2,716,582 Fat-outed Aug. 30, 1955 POSITIVEOPENING AND CLOSING SWITCH WITH WIPING ACTION Edmond G. Franklin,Minneapolis, Minn, assignor to General Mills, Inc., a corporation ofDelaware Application September 2, 1953, Serial No. 37 7,999

Claims. (Cl. 200138) This invention relates in general to electricswitches, and pertains more particularly to a switch especially adaptedfor use in conjunction with the thermoelectric control of appliances andthe like.

In the control of heating circuits, and in other 1nstances where theswitch contacts are subjected to successive or repetitious make andbreak conditions, it has been found that the life of the contacts can beprolonged by establishing what is commonly termed a "wiping action. Thewiping action or relative movement of the contacts while in engagementis responsible for the effective dislodgment of dust particles thatwould otherwise prevent good contact. Also, at times a film is apt toform on the contact surfaces and this film causes p oor electricalcontact without the aforenoted wiping action. in brief, without anywiping action the contacts Oran electrical circuit are prone to becomepitted, a condition which progresses until the contacts must bereplaced.

While the problem of pitting is well known, its economical solution hasnot been satisfactorily achieved. Accordingly, one object of the instantinvention is to provide an electric switch having a wiping action whichis obtained without resort to expensive or complicated structure.

Inasmuch as a firm pressural engagement of the contacts also aids intheir preservation, mainly due to the lowering of the electrical surfaceresistance with a concomitant lessening of heating in the contact area,it 1s an aim of the invention to provide a switch having positive makeand break characteristics as well as the above referred to wipingaction.

Another object of the invention is to delay slightly the cycling actionof the switch contacts. For example, where the invention is applied to athermal switch, the thermally responsive element is likely to fluctuatein its movements just at the moment of contact or separation which,without employing the proposed arrangement, would cause the contacts toopen and close a number of times before a more stable condition isestablished. This fluttering action in the past has resulted in thedrawing of arcs which over a period of time greatly contributed to thedeterioration of the contacts. However, by automatically increasing thecontact pressure at the moment of engagement, as envisaged by thepresent invention, this adverse fluttering is avoided. Also theinvention further contemplates an accelerated separation of the contactsonce the thermal element has permitted opening thereof, thus obviatingany objectionable fluttering during this period.

Other objects will be in part obvious and in part pointed out more indetail hereinafter.

The invention accordingly consists in the features of construction,combination of elements and arrangement of parts which will beexemplified in the construction hereafter set forth and the scope of theapplication of which will be indicated in the appended claims.

In the drawings:

Figure 1 is a plan view of the invention in the form of a thermalswitch;

Fig. 2 is an elevational view, partly in section, corresponding to Fig.1, the contacts being shown at the moment of initial contact;

Fig. 3 is a view similar to Fig. 2 but with the parts of the switchstructure shown in the relative positions they assume after the contactshave been closed for a relatively short interval of time;

Fig. 4 is an operational continuation of Fig. 3, the contacts havingjust been separated by the thermally responsive element;

Fig. 5 illustrates the switch after the contacts have remained separatedfor a relatively short period of time.

Referring now in detail to the drawings, the switch has been designatedin its entirety by the reference numeral ill and for illustrationpurposes has been exemplified as a thermal switch for controlling thetemperature of an electrically heated member 12, which may be a portionof an appliance such as an iron or wafiie baker.

To produce a unitary switch structure the switch 10 includes a tubularmetallic post 14 equipped with a flange 16 at its lower end and ariveted or staked upper end 18. Passing through the bore of the post 14is a bolt 20 having a head 22 or" sufiicient diameter to overlap theupper end 18 of the post and a threaded shank 24 engaging a tappedrecess 26 provided in the member 12.

Having one end anchored to the tubular post 14 is a bimetallic member28, the other or free end of which supports an upstanding stud 363. Aswill hereinafter become apparent, the metals constituting the bimetal 28are selected so that when the member becomes heated the stud end willdeflect upwardly.

Also included as parts of the switch is a pair of resilient metalliccontact arms 32 and 34, each provided with an electrical terminal 33 and355, respectively, adjacent its anchored end. The arm 34- has a portionthereof stamped or punched so as to form an offset contact support 36and a pair of struts 38 and 4t integral with the support 36 at one endand with the body of the arm 34 at the other end. On the support 36 ismounted one contact 42 and another contact 44 is mounted on the free endof the arm 32. By making the strut 38 of greater length than the otherstrut 4d the contact 42, as will presently be better understood, istheoretically constrained to traverse a somewhat arcuate path whensufiicient current courses through the contacts 42 and 44. In thisconnection, it will be appreciated that the comoined cross-section ofthe struts 38 and 40 is limited with respect to the magnitude of currentintended to pass through the contacts 4-2 and 44 so that substantialexpansion and contraction of the struts will ensue upon engagement anddisengagement of the contacts. Although in the preferred arrangement,only two oppositely directed struts are contemplated, as the soleconnection between the arm 34 and support 3%, it will be recognized thatadditional struts might be utilized as long as the expansion thereof onone side of the support 36 is greater than that vhich takes place on theopposite side of said support, for it is by reason of the unequalexpansion that two dimensional movement of the contact 42 relative tothe contact 44 is obtained.

in order to adjust the switch it? for operation at Various temperatures,a bracket 46 is supported at one end by the tubular post M, the otherend of which is reversely bent at dd for the purpose of mounting anadjusting screw 5'0 including a manipulating knob 52, a threaded shank54 and a lower tip of insulating material extending downwardly throughan aperture 58 in the arm 34 into abutting relation with the other arm32. Also, the screw may include a collar 60 having a radially extendingear 62 engageable with an upstanding stop 64 on the end 48 to limitrotation of the screw in either direction. By virtue of this arrangementthe turning of the knob 52 is instrumental in positioning the contactwith respect to the contact 42. For the purpose of insulating thevarious members 28, 32, 34 and 46 supported on the tubular post 14,these members have disposed therebetween several insulating washers 66.

Assuming a typical operational sequence, attention is first directed toFig. 2 in which the bimetal 28 has straightened sufficiently to permitthe resiliency of the arms 32 and 34 to bring the contacts 42 andtogether. In this make condition current will flow through the struts 38and 40 and owing to the relatively small crosssection of these strutsthey will begin to expand due to the heat generated therein by thecurrent. However, since the strut 38 is longer than the strut 4% it willhave a greater amount of expansion and the contact 42 will tend to movedownwardly and to the right as viewed in Fig. 2. Actually, in theexemplified embodiment, the arm 34 is more resilient so that it movesupwardly once the contacts 42 and 44 have met, but it will be observedby an inspection of Fig. 3 that the movement of the contact 42 to theright has produced a wiping action between the two contacts. For thepurpose of ready comparison, the relative position of arm 34 (Fig. 2)has been superimposed upon that of Fig. 3 in phantom outline.

Once the contacts 42 and 44 have remained closed for a time sufficientto permit the current flowing therethrough .1

to heat the member 12, the oimetal 28 will deflect upwardly and in Fig.4 this deflection has reached the stage where the stud 34 causes thecontacts to separate. The interruption of current results in a coolingoff of the struts with an accompanying contraction thereof, Fig. 5 il-'3'.

lustrating the struts after a short lapse of time which is sufficient toreturn the struts to their initial length, i. e., the length pictured inFig. 2, although such period of time has not permitted the member 12 northe bimetal 23 to cool to the state shown in Fig. 2. Here again, thephantom outline position affords a facile means of comparing the contactmovement between Figs. 4 and 5.

From the foregoing description, it can be readily seen that a goodwiping action occurs between the two contact positions occupied by thecontacts in Figs. 2 and 3, and

that once contact has been initially established in Fig. 2, an increasein contact pressure results due to the vertical component of strutexpansion, the struts 38 and 41 being arranged at an acute angle withrespect to the general plane of the arm 34 to accomplish this end. Ofcourse, 51

the difference in expansion between the two struts is responsible forthe horizontal displacement component of the contact 42. From Figs 4 and5, it will be discerned that the reverse action takes place uponseparation of the contacts, there being an accelerated separation of thecontacts by reason of the contraction of the struts 38 and 40 after theyhave initially separated.

It might be specifically mentioned in conclusion that it is within thepurview of the invention to utilize struts corresponding to thoselabeled 3% and 48 having equal lengths but different cross-sectionalareas, whereby the electrical resistance is made different by virtue ofthe different cross-section in contradistinction to the differinglengths as illustrated. In this latter situation the strut having thelesser cross-section and thus greater resistance would heat to a highertemperature and would therefore expand lengthwise more than the otherstrut to produce the desired wiping action. Still further, in someinstances it might be advantageous to have one strut both longer and ofless cross-section than the other, thereby providing an even morepronounced wiping action. Also, it is within the contemplation of theinvention, although not preferred because of increased manufacturingcosts, to

produce struts having different expansion characteristics by utilizingdifferent metals for the struts 38 and 49.

As many changes could be made in the above construction and manyapparently widely different embodiments of the invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the followingclaims is intended to cover all of the generic and specific features ofthe invention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

I claim:

l. A contact structure comprising an electrically conductive element, acontact offset from said element, and means mounting said contact formovement relative to said element, said means including a pair ofcurrent conducting struts inclined at an angle with respect to saidelement and capable of unequal lengthwise expansion upon the passage ofsufficient current therethrough.

2. A contact structure comprising an electrically conductive element, acontact offset from said element, and means mounting said contact formovement relative to said element, said means including a pair ofcurrent conducting struts having different values of electricalresistance to render said struts capable of unequal lengthwise expansionupon the passage of sufficient current therethrough.

3. A contact arm subassembly for a thermal switch, said subassemblycomprising an electrically conductive contact arm, a contact offset fromthe plane of said arm, and means mounting said contact for movementrelative to said arm, said means including a pair of current conductingstruts of unequal length inclined at an angle with respect to said armand capable of lengthwise expansion upon the passage of sufiicientcurrent therethrough.

4. A contact arm subassembly for a thermal switch, said subassemblycomprising an electrically conductive contact arm, a contact supportoffset from the general plane of the arm, a contact on said support, anda pair of oppositely directed struts joined to the support and said arm,the total cross-section of said struts being limited with respect to thecurrent passing through the contact and one strut being of greaterlength than the other.

5. A thermal switch comprising first and second contact arms havingfirst and second contacts respectively adapted to engage and disengageeach other, thermally responsive means engaging and moving one arm withrespect to the other and thereby engaging and disengaging said contactsin response to predetermined temperature changes, the first arm having acontact support on which the first contact is mounted, and a pair ofinclined struts arranged at an obtuse angle with respect to each otherconstituting the sole connection of the contact support to the contactarm and having a cross-section limited with respect to the currentpassing through the contacts for causing substantial expansion andcontraction of the struts by heating and cooling in response to thechanges in current carried by the struts on engagement and disengagementof the contacts, one of said struts being longer than the other to causea greater amount of expansion of said longer strut.

References'Cite'd in the file of this patent UNITED STATES PATENTS

